Quadrajet Problem

[mrcaruso]

Got a 'new' carb for my 75 Chev truck & it had no spring under the power piston. Engine kept running out of fuel except for a constant working of the accelerator pump. Put a long spring under the piston and it runs better but still won't keep steady rpm unless a constant working of the accelerator pump. Any ideas?
Thanks.........................

[Wrenchbender Ret]

Check the float level. It may be too low. If someone put this "new" carb together without a spring under the power piston, who knows what else is wrong with it.

[Userone]

if the carb has been sitting long, you might have trash in the needle seat. mine is running like crap like you're describing because of sitting

[mrcaruso]

Thanks guys. When I had the carb apart for cleaning, I raised the float level to approximately 1/4 inch. It was very low, at least 1/2inch. I replaced the needle & seat when I put in the spring under the power piston. I don't even have a filter installed so that can't be the problem. I appreciate your input.

[OutlawDrifter]

sounds like it needs to come apart and be inspected. there are a lot of parts that can go missing from a q-jet and they still look complete when put together.

do you know any history on the carb at all? did it have a kit put in it, or was it just setting on a shelf? most of the time if a q-jet is drained properly after being removed and stored well, you can throw them on and they run just like they did when they came off.

[mrcaruso]

The carb came off a 78 Chev1 ton truck. The guy said he put marvel oil in it then put it on the shelf. I took it apart and put in some parts out of a kit, but I think I may have out thought myself and raised the float level too far. My next step will be to drop it back to the 1/2 inch mark. There are so many diff float level settings motor/trans fuel pump pressure combos from 65 or so on that it's tough to determine the correct setting. This motor is a 383 with a 30-30 hydraulic cam, so vacuum could be a hurdle with the power piston as well. I love a challenge! I will post the cure, if I find it!
Thanks again.

[OutlawDrifter]

yes, that cam will have poor vacuum for sure!

the carb would probably run fine on a milder setup.

[D.PASSMORE]

There is a spring kit available. It comes with different length springs. At idle that spring keeps pressure on the power piston as the vacuum pulls the piston and rods down into the jets, restricting fuel flow (part of the idle circuit). As the engine is revved, vacuum drops, piston and rods pull out of jets and more fuel enters the intake manifold.
If you are playing with Quadrajets, I would read up on them. IMO, best carb out there, but have to be tuned properly. Not hard if you understand them.

[mrcaruso]

Thanks, David. I haven't been into a Quad since my stock eliminator racing days and I'm now studying my Rochester book again! When I took this baby apart, it had NO spring under the power piston, which I found odd. I purchased a kit and some parts which included a spring kit. i put the longest spring under the power piston and it does run better but still seems to starve for fuel. I will find the problem and let you guys know what happens. Thanks to all of you for helping me out.

[OutlawDrifter]

cliff ruggles seems to be the man in the know when i comes to q-jets.

when i was rebuilding the q-jet for my 82 K5 i bought his book and ordered the parts from his website. prices were fair and i was able to get my carb tuned dead on with his suggestions.

[mrcaruso]

Thanks for the info.

[mechanicalman]

Originally Posted by mrcaruso

Thanks for the info.

This might help on some parts that might work better than what you have. Sounds like lean jets? You may also have a vacuum leak internal or external.


For 1975-85 305-454 Chevy/GMC trucks and cars with a hot air style choke. Compatible with EGR. Not for computer-controlled vehicles. Main Jet: .073; Metering Rods: Primary-50M, Secondary-DR. Pictured with electric choke does come with hot air choke.
•795 cfm with hot air choke #1903

What are the numbers off the carb? 1904 pretty much same carb but uses an electric choke.
http://www.carburetion.com/quadnumber.htm

This site can get you just about any darned carb part you want, reasonable, I've dealt with them before and had no problems.
http://www.carburetion.com/
http://www.carburetion.com/quadrajet.asp

Get the right repair kit for it, break it down, replace any missing parts and jets, metering rods, secondary hanger/rods, etc. for a 1903 or 1904 and that will be a good starting point. You may need to alter idle air bypass if you have a big cam, if you have problems adjusting your idle mixture. Secondary air valve adjustment also helps responsiveness. Also make sure the accel pump check ball is there, they usually come with a kit. Use the blue (Viton) accel pump seal if alcohol mix fuel is used in your area for longer life. And put a filter in it, you will likely be missing the filter springs so you can get one from carbs unlimited.

Look out for the old warped air horn (top of carb). You can actually straighten them with a mallet and some blocks of wood, but be careful you might want an old carb guy to do that for you if you need it. This happens as a result of over-tightened bolts holding the carb to the intake manifold, always use a nice supple gasket and don't over-tighten.

[mrcaruso]

I have replaced the normal parts from a kit. The normal openings in the base plate are plugged or hooked to the PCV checks. There may be a vacuum leak internally that I missed. When I replaced the gaskets I matched them the old so as not to make that mistake. One thing I did do was raise the float level to approx. 1/4 inch. It was over .400 and I assumed that it was too low. That doesn't equate to what I think is not enough fuel flow, but I am going to lower it back to where it was and see if that makes a difference. I really appreciate all the information regarding the parts, gaskets and other problems.

[Fitz]

Originally Posted by mrcaruso

I have replaced the normal parts from a kit. The normal openings in the base plate are plugged or hooked to the PCV checks. There may be a vacuum leak internally that I missed. When I replaced the gaskets I matched them the old so as not to make that mistake. One thing I did do was raise the float level to approx. 1/4 inch. It was over .400 and I assumed that it was too low. That doesn't equate to what I think is not enough fuel flow, but I am going to lower it back to where it was and see if that makes a difference. I really appreciate all the information regarding the parts, gaskets and other problems.

Quadrajets are the reason I run a Holley. Never could figure one out. Must be a genetic problem or something.

[mrcaruso]

Lowered the float level and it did help, but there is definitely some sort of vacuum leak. Took it apart and checked the gaskets to make sure I installed the correct ones. I can't find any holes blocked off or plugged. The was no spring under the power valve when I initially took it apart. I had a kit of springs and put in the longest one. It is quite a bit longer, so it may be causing a 'tug of war' with the manifold vacuum causing a surge, so maybe not a vacuum leak? I will install one of the shorter springs and see if that cures the problem.
Thanks again for the info and assistance from all of you who responded to my thread.

[mechanicalman]

Originally Posted by mrcaruso

Lowered the float level and it did help, but there is definitely some sort of vacuum leak. Took it apart and checked the gaskets to make sure I installed the correct ones. I can't find any holes blocked off or plugged. The was no spring under the power valve when I initially took it apart. I had a kit of springs and put in the longest one. It is quite a bit longer, so it may be causing a 'tug of war' with the manifold vacuum causing a surge, so maybe not a vacuum leak? I will install one of the shorter springs and see if that cures the problem.
Thanks again for the info and assistance from all of you who responded to my thread.

Power piston is designed to meter fuel in proportion to engine load factor. It has little to do with cruise or stead-state RPM conditions.

Longer/stronger power piston spring means richer mixture, shorter/weaker means leaner mixture. Vacuum holds the piston down restricting fuel flow through the jets, the spring holds the piston up. When you open the throttle, vacuum drops and the piston goes up pulling the metering rods out of the jets allowing more fuel richening the mixture thus "power" valve designation. When at cruise vacuum goes back up the piston goes down and metering rods with it leaning out the mixture in proportion to load.

I don't think the power piston is the source of the problem, despite the fact it did need a spring installed. It's ironic that the lack of a spring only made the lean condition worse.

You are not getting enough fuel through your venturi circuit. You need to strip down the float body, remove the jets and the pump check ball, and soak that thing in some real carb cleaner. Check all your surfaces for flatness, blow it out with compressed air and using that air confirm all passages are patent. Before you re-assemble it, JB weld the lead plugs on the bottom of the bowl to prevent leakage (common problem) and also the JB weld the plug on the front of the carb next to the fuel side inlet and all the ones in the picture.

Start with a medium length/strength power piston spring and adjust from there. Do not try to correct the real problem by over-compensating on the power piston. Once you get it to run steady-state then evaluate fuel mixture under load and evaluate your power piston spring then but keep in mind you may have wrong parts like jet size and metering rod size.

You may have to kiss that zinc color goodbye.

The only part of that carb that can be repaired for warping is the top cover as I described.

I assume you have eliminated external vacuum leaks.

[mrcaruso]

I wasn't sure which spring was correct so I put in the longest one. I will compare springs with my old carb and change that. Both sets of plugs had JB weld on them. I'm pretty sure they are OK as when I have let the carb sit for a few days, there is fuel in the bowl when the top is removed. I will check the surfaces and follow your advice. The rear vacuum exit had been plugged, the front vacuum exit is hooked to both PVC valves in the valve covers. The other two are connected to the distributor and to the vacuum valve for the heater control. I will plug those just to check that there isn't something leaking before those connections. Good thing that I'm not married to the zinc color! Thanks for the assistance.

[mechanicalman]

Originally Posted by mrcaruso

the front vacuum exit is hooked to both PVC valves in the valve covers.

If you have 2 pcv valves, I'd eliminate one of them and run a breather on the other one.

2 pcv valves and no breather would cancel each other out and possibly cause excessive vacuum leakage.

Pcv valve sucks blowby and eliminates crankcase pressure, breather allows it to suck blowby and relieve crankcase pressure.

[Troy Sr]

One thing that I've found helped me in getting a quadrajet working on my boat was a magnifying glass with a led light and I looked into every hole in the carb looking for blockages. I found some that couldn't be seen with naked eyes.

[jlsanborn]

I have a couple great documents prepared by Lars Grimsrud on the q-jet. Tuning and troubleshooting. Should be pretty easy to find on the web, or if you PM me an email address I can send them to you. The file sizes exceed what the forum will allow to be uploaded.

[mrcaruso]

What I have done is installed a new float (.345 setting), eliminated one PVC valve, installed the shorter power valve spring and it runs much better then it did, however it stills seems to have a small vacuum leak. I disconnected the vacuum heater control line and the dist. advance and it made no difference. It did act like it was flooded a couple of times. I noticed that the fuel rods don't seem to go very far inside the jets, so the small part of the rods are still up in them which make appear that there would be no difference in the fuel going through them at idle or higher rpm.
If you are unable to send those files on the site, how can we get that done?
Thanks.........................

[davepl]

Originally Posted by Fitz

Quadrajets are the reason I run a Holley. Never could figure one out. Must be a genetic problem or something.

And Holleys are the reason I run Quadrajets. At the track I'd run a Holley, as they're dead simple. And once you figure out all the hacks like connecting the vent tubes so you don't splooge fuel on launch, they work well.

There's a reason everything up to the the LS5 was Q-Jet; properly calibrated they run -far- better than an equivalent Holley. And I better I do not mean 1/10th at the track yet you have to drive it around with two feet so it doesn't stall when cold. I just mean driving around town.

Up until the point they exceeded the airflow of the Q-Jet GM always used them. The Corvette kept 3x2 because it was cool (except the rare LS6 I guess). Later GM adapted them to 800 cfm and so on for the big 455s, but until then the Holley was your only choice on big airflow motors.

In all honesty both will make the same power when properly calibrated (on an engine matched to their size). The Q-Jet will have better driveability, cold start, warmup driving, and pretty much everything - when properly calibrated.

Unfortunately, probably only Cliff Ruggles and three surviving engineers from Rochester can do it. And it took hundreds of hours for GM to calibrate to a specific application. And float levels vary by application - there's no "right" level for all carbs.

So that's the problem - they're so complicated to get right and so easy to screw up that eventually people pull them off and stick the old firehose 850 double pumper on it.

I can't understand why people think fuel injection is complicated - I'd rather reverse engineer the assembly code in an LS3 controller than figure out all of the systems in a Q-jet. But if someone good will do it for me, great :-)

[mrcaruso]

I hear you on the Holleys. I ran a tunnel ram with 2 Holley 600s on a small block Vega and my 69 Ply Six Pak had 3 Holleys. I thought about using fuel injection on this, but I had the manifold and decided to use it and of course I was reluctant to spend $1500 plus for an aftermarket setup. I can use the challenge, but I have had some really good help here, which I appreciate very much!

[jlsanborn]

Originally Posted by mrcaruso

If you are unable to send those files on the site, how can we get that done?
Thanks.........................

PM me your email address and I will send them to you that way.

[mrcaruso]

It's paulcaruso38@yahoo.com